Due to the recent development of small sensors with the use of the MEMS (Micro Electromechanical Systems) technique, sensors for sensing an acceleration are used in various instruments such as cell phones or game machines, or the use thereof is under review. Such an acceleration sensor is manufactured by using an SOI wafer of a three-layered structure including a silicon layer, a silicon oxide layer, and a silicon layer, for example. The sensor of this type includes a frame having an opening that is formed to pass through the SOI wafer, and a weight supported on the frame through a plurality of beams, the weight being displaceable when an external force acts thereon. When the weight is displaced by an external force (acceleration) acting thereon, the displacement is sensed so that the acceleration is measured. (See, JP 2003-329702 A1 and JP 2004-144598 A1.) Sensors for measuring an acceleration by sensing a displacement are classified into piezoresistance-type sensors in which a piezoresistors are arranged on beams and a resistance change by a strain of the beams is detected, and capacitance-type sensors in which a capacitance change in accordance with a displacement of a weight is detected.
In the aforementioned acceleration sensor, there is a possibility that, when an excessive acceleration is applied to the weight, the beams are excessively displaced and that the beams and/or the weight are broken down. For this reason, an acceleration sensor for use in a cell phone or the like is required to have an impact resistance against 3000 G. This is because an impact value applied to a wooden plate having a thickness of 100 mm is 1500 to 200 G, when the acceleration sensor is pulled down by gravity from a one-meter-high position toward the wooden plate.
However, in view of a wider variety of instruments on which the acceleration sensors are mounted, the acceleration sensor is now required to have an impact resistance against as high as 5000 G or more, but there is not yet obtained such an acceleration sensor having a high impact resistance.